I'm assuming you mean ACEA A1 & A5. It is as follows: "Oil for gasoline engines specifically designed to use low friction, low viscosity oils with High Temperature/High Viscosity of 2.6 to 3.5 mPa.s. May be unsuitable for use in some engines".
ACEA A5 "Stable stay-in-grade oil intended for use at extended drain intervals in high performance gasoline engines designed to be capable of using low friction, low viscosity oils with a HT/HS of 2.9 to 3.5 mPa.s. These oils may be unsuitable for use in some engines".
It's not that I'm that smart. I found these specs not too long ago and saved them for reference. Maybe that was smart.
[ July 23, 2004, 09:09 PM: Message edited by: krholm ]
Energy-saving, stable viscosity (stay in grade) category with stricter requirements. Developed especially for high specific power engines operating with low friction, low viscosity (2.9 mPas < HTHS < 3.5 mPas) engine oil. Suitable for long oil change intervals approved by the manufacturer.
Not for use with certain engine types.